The invention relates to an electric machine tool for tools which operate by rotation and/or percussion, such as a drill and/or riveting hammer or impact drilling machine.
In connection with a known drill and/or riveting hammer of this type (DE 41 21 279 A1), the drive unit comprises an eccentric seated on a gear shaft, which drives the work spindle via a gear wheel and, via a needle bearing, receives a coupling sleeve having an opening, as well as an elastically yielding driver member, which is seated, tiltable around an axis oriented transversely in relation to the gear shaft, in the machine housing. The driver member has a lever extending away from the axis toward the coupling sleeve, which engages the opening in the latter, and a two-legged hoop extending away from the axis, which is closed on its free end in a loop-like manner and is hinged with play between two collars formed on the beater. The beater is received with displacement play in the hollowly embodied drive spindle, wherein an inserted O-ring acts as a damper on the beater and prevents it from being displaced on its own. A header or a riveting bolt has been inserted between the tool shaft and the beater.
During operation, the driver member is driven via the eccentric in a back-and-forth movement, wherein only the vertical excursions of the eccentric are transmitted to the lever, while the transverse movements of the anchor sleeve do not reach the lever because of the opening, which is widened in this direction. Accordingly, the driver member performs a back-and-forth movement around its axis. At the moment of the impact of the beater on the header, and therefore on the tool, the driver member is at dead center on the tool side. Following the strong impact, the beater is reflected and flies backward toward the hoop of the driver member, which also moves backward. When the striking mechanism is well adjusted, the front collar touches the hoop of the driver member only slightly, or not at all. After passage through dead center on the motor side, the driver member again comes into contact with the front collar of the beater. Because of the kinetic energy of the beater, the hoop is bent backward in the process. Thus, the energy still stemming from the recoil of the beater is transferred to the elastic driver member and stored therein as spring energy. In the subsequent forward movement of the hoop, the latter accelerates the beater in the direction toward the tool, both because of the forward movement of the driver member and because of the backward springing hoop, wherein as a rule the beater attains higher velocities than the driving hoop. This leads to the separation of the beater from the driver member. The beater then flies freely over a defined distance, until another impact on the header and the tool takes place.